Recently, flat panel display devices, such as liquid crystal display devices, have been well developed as substitute components for cathode ray tubes. Such liquid crystal display devices have significant advantages, e.g., light weight, thin thickness, low power consumption, etc. Their mainstream products are active matrix type liquid crystal devices because their display images have substantially no cross-talk between neighboring pixels due to thin film transistor switching elements connected to pixel electrodes. Thus, by way of example, such active matrix type liquid crystal display (“AMLCD”) devices are explained below in detail.
The AMLCD device includes a thin film transistor (“TFT”) array substrate, a counter substrate provided opposite to the TFT array substrate, alignment layers coated on opposing surfaces of the TFT array and counter substrates, and a liquid crystal layer held between the alignment layers. The TFT array substrate is provided with a transparent insulation substrate, signal and scanning lines, pixel electrodes, and thin film transistors. The insulation substrate is made of glass, quartz, or the like. The signal and scanning lines are disposed in a matrix form and are isolated through an insulation layer. The pixel electrodes are provided at pixels and are made of transparent materials, e.g., indium-tin-oxide (“ITO”) films, etc. The thin film transistors are disposed in the vicinities of intersection points of the matrix to carry out switching functions, so that the transistors electrically isolate turning-on pixels from turning-off ones and hold video signals supplied to the turning-on pixels. The gate and drain electrodes of the transistors are connected to the scanning and signal lines, respectively, while source electrodes of the transistors are coupled to pixel electrodes.
The counter substrate, on the other hand, includes a transparent insulation substrate also made of glass material or the like and an ITO counter electrode formed on the insulation substrate.
The AMLCD has a display region and a non-display region around the display region to define an electrically connecting portion. The non-display region projects from the display region to form a shelf-like connecting plate where connecting pads and terminals are disposed to receive input signals from external driving devices. A liquid crystal layer is held between the TFT array and counter substrates. Fringes of the TFT array and counter substrates are sealed by sealant materials.
The production cost of the AMLCD device depends highly on that of the TFT array substrate because a lot of production processing steps are necessary to manufacture the TFT array substrate. Thus, the former is a key factor to reduce the total production cost of an AMLCD device or the like.
In that connection, Japanese Patent Disclosure Tokkaihei 9-160076 proposes the following method of manufacturing a TFT array substrate: (1) signal lines, source and drain electrodes, semiconductor layers, etc., have been processed together by a same mask pattern, (2) contact holes to connect source electrodes to pixel electrodes and outer peripheral contact holes to expose connecting terminals for signal and scanning lines are perforated at the same time, and (3) pixel electrodes are formed on the top layer of the TFT substrate. Such a method can improve productivity because the number of mask patterns decreases with the yield rate substantially unchanged. Where the connecting terminals for the scanning lines, etc., however, are exposed, it is necessary to perforate holes in both interlayer and gate insulation films. Thus, a wet etching process of buffered hydrofluoric acid (“BHF”) etc. is carried out to perforate the interlayer and gate insulation films made of silicon oxide and silicon nitride, respectively, at the same time (disclosed in Japanese Patent Disclosure Tokkai 2000-267595).
The TFT array substrate of this sort, on the other hand, requires a greater aperture ratio of pixels to make the efficiency of light from a rear light source improved. Reflection type flat panel display devices, however, require an effective reflection ratio of the ambient light by increasing an area ratio of the pixel electrodes.
Recently, in order to improve the pixel aperture and reflection ratios, the pixel electrodes are formed over wiring patterns, and the thin film transistors on the TFT array substrate and a thick resin insulation film are disposed between the pixel electrodes and wiring patterns. The thin film transistors and the signal and scanning lines are disposed, also through the thick resin insulation film, at peripheral portions of the pixel electrodes. The thick resin insulation film is generally 1 μm to 10 μm and, typically, 2 μm to 4 μm in thickness and is made of a low dielectric constant organic material, so that substantially no electronic capacitor or short circuit may take place between the pixel electrodes, the signal lines, or the like.
Further, light shielding films have been integrated on the TFT array or counter substrate to cover the thin film transistors, gaps between peripheral portions of the pixel electrodes and the signal lines and those between the peripheral portions of the pixel electrodes and the scanning lines. That is to avoid undesirable electronic capacitors or short circuits due to overlaps of the pixel electrodes with the signal or scanning lines, and also to adjust discrepancies between patterns of the pixel electrodes and those of the signal or scanning lines to sufficiently prevent light from leaking between the above-stated gaps.
The thick resin insulation film resolves losses of the pixel apertures resulting from alignment margins so that the aperture ratios are improved and larger. A TFT array substrate used for reflection type liquid crystal display devices includes reflection type pixel electrodes made of aluminum or the like and formed on the top of a TFT array pattern, and a thick resin insulation film disposed between the pixel electrodes and lower wiring layers. A resulting resin layer makes peripheral portions of the reflection type pixel electrodes possible to cover scanning and signal lines and thin film transistors. As a result, since the pixel electrodes become bigger in area, the light utilization efficiency is improved. Further, the thick resin layer prevents parastic capacitors from increasing their electric capacities. Further more, it is configured to make both surface of the pixel electrode and liquid crystal layer thickness uniform.
Recently, as display performances required for compact information terminals or mobile phones become improved, transflective or hybrid (transparent and reflection) type display devices have been in use. It includes pixel electrodes made of an optically transparent and electrically conductive film (e.g., ITO film), and optical reflection type electrodes. Under lighted environment, such as sun light, the optical reflection type electrodes (reflection type pixel electrode portions) primarily perform a display function by reflection of incident ambient light but under dark environment the optically transparent and electrically conductive film carries out another display function by a rear light source.
Such transflective type display devices need the pixel electrodes consisting of two kinds of electrically conductive films formed by different patterning processes. Thus, it is necessary to carry out additionally at least one patterning process called a photo engraving process (“PEP”) in comparison with reflection and not transflective type display devices, i.e., the steps of preparing a series of additional mask patterns, photoresist resin coating, development, etching, removing photoresist resin coating and washing must be made accordingly. It results in an additional burden of manufacturing steps and an increase in production cost.
In order to reduce the number of such steps a pattern of the thick resin isolation film, for instance, may be used for a mask pattern as it is to perforate a gate isolation film and the like to make holes, i.e., contact holes of the thick resin isolation film are provided to be consistent with those of the gate insulation and the like.
In this case, however, side etching or the like of the gate insulation film makes overhanging portions and causes discontinuity between electrically conductive films coating the contact holes (i.e., a rift or break at a step defined between conductive films).